leedan
Mechanical
- Nov 27, 2012
- 3
Hi all. I have frequented this site for tips and suggestions for questions I have and it has been very helpful. This is my first time posting a question. I have searched the forums to see if there is anything related to this, but I haven't been able to find something that would guide me in the right direction.
I am trying to verify that the relief valve that the vendor supplied will be sufficient as a thermal relief valve. The current pipe configuration that I am trying to identify is centered around an electric circulation heater with an upstream and downstream isolation/bypass valves. The vendor has supplied a relief valve for the heater.
A coworker has specified there are two cases we need to determine if the relief valve that was supplied is sufficiently sized – 1) Fire Case, 2) If the upstream and downstream valve was closed and the heater was left on and safeguards to stop the heater from overheating failed and heater is constantly supplying heat.
Case 1) was easily evaluated by following API 521 fire case, but case 2) I am having some troubles. Specifically finding the mass flow rate.
I thought I would use the following equations:
1) A = m / (C*Kd*P1*Kb*Kc) * (T*Z/M)^(0.5) from API 520
a. In this case m = mass flow rate
b. I can determine the required effective discharge area and compare it to the manufacturer’s discharge area to confirm if it is sized properly.
2) Q = m * Cp * (T2 – T1) where
a. Q = Heat Duty of the heater [kW] (Which I have from the manufacturer’s data sheet.)
b. m = mass flow rate (I would be solving for this mass flow rate to input into equation 1.)
c. Cp = specific heat of the gas
d. T2 = relieving temperature (in this case the temperature where pressure of gas reaches Set Pressure + Overpressure)
e. T1 = 60 Degree F (I’m assuming this should be atmospheric temperature)
I have a gut feeling that I’m not approaching this with the right equation or the right assumptions to determine the flow rate, but I’m not sure how else I should proceed. It seems like I should be looking at the gas expansion rate based due to heat input, but I’m not sure what equations to use.
Any suggestions on how to approach sizing this thermal relief valve would be greatly appreciated, thanks!
I am trying to verify that the relief valve that the vendor supplied will be sufficient as a thermal relief valve. The current pipe configuration that I am trying to identify is centered around an electric circulation heater with an upstream and downstream isolation/bypass valves. The vendor has supplied a relief valve for the heater.
A coworker has specified there are two cases we need to determine if the relief valve that was supplied is sufficiently sized – 1) Fire Case, 2) If the upstream and downstream valve was closed and the heater was left on and safeguards to stop the heater from overheating failed and heater is constantly supplying heat.
Case 1) was easily evaluated by following API 521 fire case, but case 2) I am having some troubles. Specifically finding the mass flow rate.
I thought I would use the following equations:
1) A = m / (C*Kd*P1*Kb*Kc) * (T*Z/M)^(0.5) from API 520
a. In this case m = mass flow rate
b. I can determine the required effective discharge area and compare it to the manufacturer’s discharge area to confirm if it is sized properly.
2) Q = m * Cp * (T2 – T1) where
a. Q = Heat Duty of the heater [kW] (Which I have from the manufacturer’s data sheet.)
b. m = mass flow rate (I would be solving for this mass flow rate to input into equation 1.)
c. Cp = specific heat of the gas
d. T2 = relieving temperature (in this case the temperature where pressure of gas reaches Set Pressure + Overpressure)
e. T1 = 60 Degree F (I’m assuming this should be atmospheric temperature)
I have a gut feeling that I’m not approaching this with the right equation or the right assumptions to determine the flow rate, but I’m not sure how else I should proceed. It seems like I should be looking at the gas expansion rate based due to heat input, but I’m not sure what equations to use.
Any suggestions on how to approach sizing this thermal relief valve would be greatly appreciated, thanks!
